Media Player no play!

Hi There
Problem: I have a new Dell XPS 420 system, video card Nvidia Geforce 8800GT, Vista with Windows Media Player 11. Recently WMP stopped working - the audio plays but not the video (black screen only)! Other video players work fine - I can play the same files fine with DIVX or VLC. Streaming video via Youtube etc is OK.
Fixes tried: I have downloaded a bunch a codecs, updated the drivers for the video card and the sound card. Still no good!
Another possibly related problem is the Windows Media Centre has stopped playing TV input. I can only replay old files not current via the tuner.
Also when I run Windows update it fails to run 2 updates repeatedly: Msoft Office update (failure KB946691) and MS Office Outlook 2007 junk email filter (KB949037).
Can anyone help me get WMP going again???
Many thx
Ferris